Blowing agent

Blowing agents have been used to obtain foamable compositions of a variety of thermoplastic materials (1). 

Blowing agents work by expanding a thermoplastic resin to produce a cellular thermoplastic structure having far less density than the resin from which the foam is made. Bubbles of gas form around nucleation sites and are expanded by heat or reduced pressure or by a chemical reaction in which a gas is evolved. 

A nucleation site is a small particle or conglomerate of small particles that promotes the formation of a gas bubble in the resin. Additives may be incorporated into the resin to promote nucleation for a particular blowing agent and, consequently, achieve a more uniform pore distribution. 

Blowing agents may be physical blowing agents or chemical blowing agents (3). 

Major blowing agents appearing in the literature are listed in Table 6, and Tables 7A through 7D. 

Chemical Blowing Agents. The conventional gas-generation reaction for flexible urethane foams is the water-isocyanate reaction which was first described in a German patent (122). Its chemical reaction is shown as follows  

This reaction is carried out in two stages, i.e., carbon dioxide gas-generation, with the simultaneous formation of the substituted urea linkage. 

Unconventional gas-generation reactions have been reported by Ashida (33). The blowing agents used include the following compounds (a) enolizable compounds such as nitroalkanes (nitroethane, nitropropane) aldoximes (acetaldoximes), nitrourea, acid amides (formamide, acetamide), active methylene-containing compounds, (acetylactone, ethyl acetoacetate), and (b) boric acid. The mechanisms of their gas-generation reactions are also discussed (33). 

Recently, Speranza disclosed new blowing agents—carboxyl- 

Solid blowing agents are materials which decompose to release gases at particular temperatures matching the appropriate melt viscosity necessary to retain the foam structure. There are two main types  

PVC-U foams incorporate sodium bicarbonate and the results are available of an investigation into controlling the rate of gas evolution and heat of decomposition (281). 

The manufacture of cellular PVC/wood composites has been studied. The properties achieved, as foam density was reduced, were examined showing suitability for many wood replacement applications (196). 

A study of the effects of several different plasticisers on the density, elasticity and degree of expansion of foams, produced from different PVC plastisols, has been reported (120). A three-dimensional and high resolution quantitative image technique has been proposed for the investigation of the internal microstructure of foams. This provides a tool to study the relationships between foam structure and physical properties (18). 

The mechanical foaming of PVC pastes has been carried out for some time on the basis of air or gas being whisked into specially formulated plastisols, which are cured using microwave or infrared ovens. The lightweight material is used for sound deadening or domestic applications. There has been a relationship established between the effects of residual emulsifier in the E-PVC resin grade and foaming behaviour (101). 

Chemical blowing agents are compounds that decompose to liberate a gas in situ such as carbon dioxide, while physical blowing agents are volatile compounds that are soluble in molten polyethylene but simply vaporize after heating and when exposed to reduced pressiue, as molten polyethylene is exposed to the atmosphere as it exits the fabrication process. 

There are two main types of blowing agent used with polymer systems in general  

With plastics, the most widely used type are the physical blowing agents. 

The reactivity of the chemical blowing agent type can make analysis difficult (e.g., they will decompose in the injection port of a GC-MS instrument), and the approach that is used is to indirectly identify them by the determination of breakdown fragments. The GC-MS instrument is the best choice for this and a library of typical breakdown fragments is required. 

Examples include azo compounds, nitroso compounds, sulfonyl hydrazide compounds and sodium bicarbonate. 

Through the addition of suitable chemical blowing agents to PP, fine-celled foams with apparent density down to 0.6 g/cm can be produced by conventional extrusion or injection moulding. Injection-moulded foamed articles produce a very fine-celled 

Extruded, foamed PP film and tape can be stretched and thermoformed for trays for meat applications, cups for drinks, tape yams for carpet backing. Within the specified loading limits for blowing agent, the approval of PP for food applications is not compromised. 

The chemistry of sulfonyl hydrazides and azides has been discussed and both derivatives are used as blowing and cross-linking agents in the manufacture of foam rubbers and plastics. Some of the best hydrazides for this purpose are diphenyl ether-4,4 -disulfonyl hydrazide 59 together with the corresponding thio analogue 60. These hydrazides are respectively prepared by reaction of diphenyl ether or diphenyl sulfide with excess chlorosulfonic acid as previously described (Chapter 4, p 74, 76), followed by condensation of the appropriate disulfonyl chloride with excess hydrazine hydrate.  

Kalir and H.H. Kalir, Biological Activity of Sulfonic Acid Derivatives in The Chemistry of Sulfonic Acids, Esters and their Derivatives, S. Patai and Z. Rappoport (eds), Wiley, Chichester, 1991, 767. 

Fang and T. Jiang, Huadong Ligong Daxue Xuebao, 1988, 24(3), 286 Chem. Abs., 

Cremlyn, Agrochemicals Preparation and Mode of Action, Wiley, Chichester, 1991. 

It is possible to use DSC to study the performance of both these types, as the chemical ones will produce an exotherm in the heat capacity trace and the physical ones an endotherm as they remove heat from the system to volatilise. The trace obtained in both cases will enable the type ofblowing agent present to be deduced, by the position of the event and by reference to traces produced from standard materials, as well as the amount ofblowing agent, again by comparison with data obtained on standards. 

However, an important consideration with the chemical types is that in addition to an exotherm resulting from the chemical breakdown of the compoimd, unless steps are taken to seal the system there will also bean endotherm due to the hberation of the gas. This loss of heat from the system will cancel out some of the exotherm and interfere with the data. The use of high-pressure, sealed pans is therefore very important in studies involving chemical blowing agents as these ensure that no volatiles can escape during the analysis. 

Moulinie and Woefle [97] have described how DSC can be used to study the kinetics of the decomposition of azodicarbonamide (AZDC) in polyolefin foams. Activation energies and rate constants were computed from the results that were obtained using variable heating rate experiments. 

The advent of modulated temperature DSC (also described as alternating or oscillating DSC) over 15 years ago offered advantages to polymer analysts, in that its ability to separate the heat flow into its reversing (heat capacity related) and non-reversing (kinetic) events improved the quality of the data that could be obtained by DSC and hence increased its usefulness [98, 99]. 

Addition of a small amount of water produces carbon dioxide by reaction with an isocyanate compound. The produced carbon dioxide is released and produces a cellular structure. Other classes of blowing agents such as sodium bicarbonate. 

It is thus pertinent from this short review that catalysts and fillers have a very high affinity, and their interaction may increase or decrease polymerization rate as well as change the molecular structure of the resultant polymer. 

Many polymers are used in a cellular form in which the polymer matrix is filled 

Blowing agent Class Volatiles produced Decomposition range (°C) Gas yield (cm /g) Comments 

Azocarbonamide (I) Carbonamide N2, CO, CO2 190-230 220 Most widely used blowing agent in PVC and polyolefins. High decomposition temperature reduced by a variety of metal salts and oxides such as lead carbonate, lead phosphite and zinc oxide. High gas yield. Reaction products show little odour or discoloration.  

Dinitrosopentamethylene- tetramine (II) Nitrosoamine N2 N0,H20, CH,NH2 160-200 210 Widely used in natural and synthetic rubbers. Some use in polyolefins. 

Benzenesulphonohydrazide (III) Hydrazide N2,H20 146 170-250 Blowing power affected by phthalate and phosphate plasticisers. 

Since poly(phenylene ether)-based resins have a lightweight and are excellent in impact resistance as compared with metal or glass, the resins have been employed in a variety of fields including automobile parts, household electric appliance parts, and office automation equipment parts. However, polyCphenylene ether) resins have a bad moldabiUty. Therefore, the resins 

However, the incorporation of the poly(styrene)-based resin, which is more flammable than the poly(phenylene ether)-based resins, lowers the heat resistant temperature of the mixed resin of poly(phenylene ether)/ poly(st5n ene) and makes the resin flammable. Therefore, a novel method that enables molding a poly(phenylene ether)-based resin without incorporating poly(styrene) has been desired. Moreover, it has also been desirable to develop a method of achieving both moldability and flame resistance at the same time. 

Several rheological aspects of thermoplastic foam extrusion have been described as a phenomenological model of the flow in an extrusion die, growth and physical properties of thermoplastic material with respect to extrusion foaming. 

PVC and polyurethane constitute together the majority of expanded plastics, but expanded polystyrene packaging is also important because it has excellent protective qualities for the transport of delicate articles, and is widely used in the catering industry. Crosslinked polyethylene foams are also important and there are several other commercially successful varieties. 

Blowing agents are additives which generate a gas during the period when the polymer is beginning to solidify. The polymer solidifies when 

The use of blowing agents is not the only way to make a cellular or foamed polymer. Other methods include the mechanical generation of a froth, or the direct incorporation of large quantities of thin-walled, hollow microspheres (see the entry Hollow microspheres, in this book). 

The solubility of a gas in a polymer is highly pressure and temperature dependent. When the pressure falls, the gas becomes much less soluble and a phase separation results, with the gas forming cells. The process of evaporation counteracts the heat produced by the exothermic 

Where the blowing agent is a solid or a liquid, the amount of blowing agent used ranges from less than 1% up to about 15%, depending on circumstances. 

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Blood sugar Blood urea nitrogen Blowers Blowing agent... 

Several more recent variations of the film-to-fiber approach result in direct conversion of film to fabric. The film may be embossed in a controlled pattern and subsequently drawn uniaxiaHy or biaxiaHy to produce a variety of nonwoven products (47). Addition of chemical blowing agents to the film causes fibrillation upon extmsion. Nonwovens can be formed directly from blown film using a unique radial die and control of the biaxial draw ratio (48)... 

Diethyl Ethylphosphonate. A Hquid compound introduced for appHcations similar to those of DMMP is diethyl ethylphosphonate [78-38-6] CgH O P. This material is claimed to be less susceptible to undesirable interactions with haloaHphatic components, such as blowing agents, or with amine catalysts. 

The use of CFCs as foam blowing agents has decreased 35% from 1986 levels. Polyurethanes, phenoHcs, extmded polystyrenes, and polyolefins are blown with CFCs, and in 1990 the building and appHance insulation markets represented about 88% of the 174,000 t of CFCs used in foams (see Foamed plastics). 

CFC-114 blowing agents and refrigerants HCFC-124 HCFC-142b blends / azeotropes HFCs... 

Chemical Stabilization. The chemistry of the system determines both the rate at which the polymer phase is formed and the rate at which it changes from a viscous fluid to a dimensionally stable cross-linked polymer phase. It also governs the rate at which the blowing agent is activated, whether it is due to temperature rise or to insolubilization in the Hquid phase. 

The type and amount of blowing agent governs the amount of gas generated, the rate of generation, the pressure that can be developed to expand the polymer phase, and the amount of gas lost from the system relative to the amount retained in the cells. 

The thermal conductivity of a cellular polymer can change upon aging under ambient conditions if the gas composition is influenced by such aging. Such a case is evidenced when oxygen or nitrogen diffuses into polyurethane foams that initially have only a fluorocarbon blowing agent in the cells (32,130,143,190,191,198-201). 

Fillers (qv) are occasionally used in flexible slab foams the two most commonly used are calcium carbonate (whiting) and barium sulfate (barytes). Their use level may range up to 150 parts per 100 parts of polyol. Various other ingredients may also be used to modify a flexible foam formulation. Cross-linkers, chain extenders, ignition modifiers, auxiHary blowing agents, etc, are all used to some extent depending on the final product characteristics desired. 

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